1. Field of the Invention
This invention relates to a method of setting up bias for a magnetic tape recorder.
2. Description of the Prior Art
In magnetic recording-reproducing apparatus such as a tape recorder, it is required that alternate current bias be optimally set up depending on the kind of magnetic recording medium such as magnetic tape which is used therewith. To meet such a requirement, there has heretofore been proposed an arrangement which is so designed that a desired one of several different bias values can be manually selected. With such a conventional arrangement, however, it is essential for the operator to previously determine which one of those different bias values is suitable for the magnetic recording medium such as a magnetic tape in use; this may cause erroneous manipulation, and thus the operator is required to have a high level of technical skill. Moreover, a number of bias values are needed in order that any desired type of magnetic recording medium may be utilized, and difficulties have been experienced in an attempt to construct a change-over mechanism capable of covering such a number of bias values to be selectively set up.
Heretofore, the following methods have been proposed to set up bias values for a magnetic medium:
(1) A method wherein a signal of a constant level having a relatively low frequency (usually, 333 Hz or 400 Hz) is used as recording signal, a bias value (level) superimposed on the recording signal is increased or decreased to cause the signal recorded on the recording medium to have a maximum level, and the bias value (maximum sensitivity bias) at which the foregoing maximum level of the recorded signal is achieved is set up as the optimum bias value.
(2) A method wherein a signal of a constant level having a relatively high frequency (for example, 6.3 KHz or 10 KHz, etc.) is used as recording signal, a bias value is applied which is higher than that (maximum sensitivity bias) at which the signal recorded on the magnetic recording medium is caused to have the maximum level when the bias value superimposed on the recording signal is increased or decreased, and the level of the signal recorded on the magnetic recording medium is made to be a constant level which is attenuated at a predetermined rate with respect to the aforementioned maximum level. The recording signal mentioned above is of a relatively low level (generally, about 20 dB down from the reference level).
(3) A method wherein two signals are used as recording signals, one of which is of a relatively low frequency (for example, 333 Hz) and the other of which is of a relatively high frequency (for example, 8 KHz), and the bias value superimposed on each of the recording signals is increased or decreased as in the above-described method (2), and an optimum bias value is set up so as to control the difference between the levels of the recording signals at which maximum magnetic change quantity is produced in the magnetic recording medium by each of the recording signals. The "maximum magnetic change quantity" referred to above corresponds to "maximum operational level", which means the recording signal level at which the third harmonic distortion of the recorded signal having the aforementioned relatively low frequency (for example, 333 Hz) reaches 5%. (This can also be said about the signal having the above-mentioned relatively high frequency (for example, 8 KHz), but the third harmonic distortion of such a signal is as high as 24 KHz and thus it cannot be reproduced, the level of which is substantially equal to the maximum output level of the 8 KHz signal.
With the method (1) in which only a low frequency signal is used, however, it may sometimes happen that in the case of a high quality tape, a suitable bias value cannot be achieved for high-frequency recording by means of bias setting related to such a low frequency alone.
By the method (2), the foregoing drawback of the method (1) can be eliminated since a recording signal of a high frequency is used therein; however, the method (2) has the drawback that a suitable bias value cannot be achieved with respect to high-input operation or low-frequency operation, due to the fact that the recording signal level is relatively low.
In the method (3), as mentioned above, to eliminate the aforementioned drawbacks of the methods (1) and (2), two signals, one of which is of a low frequency and the other of which is of a high frequency, are used, and as the recording signal level, use is made of a level in the vicinity of the maximum operational level. Thus, the method (3) is suitable for setting-up of bias. However, it is disadvantageous in that it requires very complicated manipulation and a variety of measuring instruments and is time-consuming and expensive in respect of the setting-up operation.